The burning of solid fuel, in particular, coal, in a boiler or furnace is one of the most wide-spread methods of generating heat and/or energy. With the increasing costs of fuel, in particular, coal, maximizing the efficiency of boiler performance has become a paramount goal.
One parameter that effects the BTU's that a boiler requires, and its performance, is the moisture content of the coal. Moisture is present in commercial feedstocks as both inherent and surface moisture. Inherent moisture generally does not vary outside a narrow range, while surface moisture may vary greatly. The presence of moisture in the coal produces two types of errors or fluctuations which decrease boiler efficiency. The first is a weighing error. Generally, coal feeders are gravimetric in nature, the amount of coal being delivered to the feeder being tied to the calculated mass of the fuel source. As the moisture content in coal increases, an increasing portion of the fuel delivered is not in fact BTU releasing fuel at all, but water. As BTU content decreases, a BTU imbalance in the boiler occurs, with changes in operating conditions.
The presence of this moisture introduces a second error and further decreases boiler efficiency. There is a substantial heat loss encountered in changing the moisture in the coal into steam (1030 BTU/lb. of H.sub.2 O). Accordingly, among other variables, the variation of surface moisture content of coal requires a boiler control system to correct for variations in the BTU content of the fuel, and the oxygen, in the form of air, necessary to completely combust the available coal, thereby maximizing the release of BTU's available.
As noted, in conventional systems, the amount of fuel supplied is generally determined on a mass basis, which does not allow for compensation for the moisture content of the coal.
BTU release is achieved by combustion of the combustible elements in the fuel through rapid combination with oxygen, in the form of air feed to the boiler. The air supplied, like the mass of the fuel supplied, is subject to control systems to maximize the heat released while minimizing losses. Some of the losses are the result of incomplete combustion, which forms CO rather than CO.sub.2, and the losses going up the stack. The failure to convert CO into CO.sub.2 results in appreciable loss in combustion efficiency, since only 28% of the available energy in the carbon is released.
Thus, one of the controlled system functions is to provide enough air to ensure complete combustion while keeping the amount of air in excess of that theoretically required for perfect combustion to a minimum. The excess air not used in the combustion of a fuel unit leaves the unit at stacked temperature. The energy required to heat this air serves no purpose and is lost, reducing boiler efficiency.
In addition, the amount of NO.sub.x (a pollutant) in the flue gas is a function of the excess air in the boiler. Theoretically, if there is no O.sub.2 available to bond with nitrogen (N), there is no chance for the formation of NO.sub.x. As noted above, the BTUs available, and accordingly, the overall amount of air necessary, will vary with moisture content. Accordingly, some excess air is necessary to ensure release of all the energy in the fuel, but, at the same time, excess air reduces the boiler efficiency and contributes to air pollution. An ideal combustion control system, or feeder system, should be designed to match the feed available BTUs with an appropriate amount of air, and thereby maintain the BTU level at as constant a value as possible.
Conventional prior art boiler control systems generally control boiler performance and efficiency by adjusting the air flow to the boiler according to the measured temperature or pressure of the fuel load, and/or adjusting the amount of fuel being feed to the boiler. Such systems are described in U.S. Pat. No. 4,313,387 and Canadian Pat. No. 465,659. Other systems measure the same variables, but compensate by adjusting the fuel load delivered to the boiler, as is done in U.S. Pat. No. 4,071,166. Conventionally, these control systems may be combined, so that the amount of air delivered and mass of fuel delivered to a boiler is constantly adjusted, based on measurements of the temperature of the fuel load, combustion gases and pressure.
A conventional, simplified boiler control system is illustrated in FIG. 1. Fuel demand is calculated on the basis of measurements made of boiler performance, after burning of the fuel. This calculated fuel demand signal is communicated to the fuel feeder, thereby increasing the fuel flow and the net BTU available.
In this system, steam flow is equivalent to turbine output and steam pressure to the energy (BTU) imbalance in the system. The steam flow signal sets the fuel firing rate for the correct value at a steady state. This signal is corrected by a function generated to match the efficiency v. load characteristics of the boiler. On any load change, the steam pressure loop generates an over-or-under firing signal to the fuel control in order to move the unit as rapidly as possible to the new load level. As is apparent from FIG. 1, correction to both the fuel supply and air supply system of the boiler is dependent on measurements taken during and after burning of the fuel already supplied to the boiler, for correction for changes in the fuel load, which may be due to changes in the moisture content. Generally, these measurements are made of the fuel bed, or, more popularly, of the effluent gases of the boiler. Accordingly, as illustrated in FIG. 1, the fuel flow is not adjusted for changes in the fuel load, nor is the air stream adjusted, until after the fuel originally causing the load imbalance to occur is burnt.
A wide difference in the amount of correction required by the fuel in air controls occurs with changes in the BTU content of the fuel supplied to the boiler. The compensation in the fuel loop in the control system of FIG. 1 is carried on initially by changes in the firing rate signal until the BTU correction loop takes over. The function of the BTU correction loop is to compensate for any difference between the inferred BTU content of the fuel and its actual net value which, as noted it above, may be different due to changing moisture content. The fuel loop is, in theory, a BTU loop which satisfies the system heat input requirement; but, in reality, it is a loop that supplies pounds of fuel with an inferential heat value and to which a correction signal is applied to compensate for errors in this value. As noted, the BTU correction value is determined by measurements taken from the boiler after burning of the fuel, i.e., steam pressure. The disadvantage of this system is that the BTU correction loop has a slow response and substantial BTU imbalance in this system can develop when a change in the BTU content of the fuel occurs, for instance, upon a change in moisture content.
It is apparent that these control systems compensate for BTU variations in the fuel while or after the fuel is burnt. These systems eventually restore the BTU/air balance in the system, however, while correction is being effected, frequently a period of several minutes, a BTU/air mismatch occurs, since the air is not set at the correct value for the BTUs available. Thus, another reason for maintaining an excess air cushion in the boiler is to prevent an unsafe, fuel-rich mixture during transient conditions. If a means to determine the BTU value were immediately available for the central system of FIG. 1, a heat imbalance could be avoided when the BTU content of the fuel changes. Thus, if a means to detect and compensate for BTU variations in the fuel before it is burned could be employed, the BTU/air mismatch could be eliminated or at least substantially reduced in magnitude and duration. This would allow for operation of the controlled boiler with a reduced excess air cushion. As noted, the presence of an excess cushion reduces boiler efficiency. A reduction in the amount of this cushion necessarily improves boiler efficiency.
It is an object of this invention to generate a moisture compensated dry weight coal signal for use in a boiler control system.
It is another object of this invention to provide a dry coal weight signal generating system upon which a boiler control system which improves boiler efficiency can be based.
It is another object of this invention to provide a boiler control system which compensates for the surface moisture content of the coal being feed.
This and other objects of the invention will be apparent from the detailed description below.